This invention relates to lithographic printing plates with visible images and to a process for making such plates.
Lithographic printing techniques, using, for example, anodized and silicated aluminum base plates such as described in Fromson U.S. Pat. No. 3,181,461 issued May 4, 1965, have come into widespread use in the printing industry and especially in offset printing and direct lithographic printing by newspapers using converted letterpress printing presses.
A conventional negative working lithographic printing plate of this type has a coating of a light sensitive substance that is adherent to the aluminum base sheet for exposure. If the light sensitive coating is applied to the base sheet by the manufacturer, the sheet is referred to as a "presensitized plate". If the light sensitive substance is applied to the base by the lithographer or trade plate maker, the plate is referred to as a "wipe-on plate". Depending on the nature of the photosensitive coating employed, a coated plate may be utilized to reproduce directly the image to which it is exposed, in which case it is termed a positive-acting plate, or to produce an image complementary to the one to which it is exposed, in which case it is termed a negative acting plate. In either case, the image area of the developed plate is oleophilic and the non-image area is hydrophilic.
In the case of negative working plate, the surface is coated with an aqueous solution of a conventional diazo resin. The plate is dried and exposed through a negative. The exposed image areas become water insoluble and the unexposed non-image areas remain water soluble. The plate is conventionally developed with a lithographic lacquer which consists of a two-phase system, one phase containing an oleophilic resin in a solvent and the other phase a hydrophilic gum. Upon application, the oleophilic resin adheres to and makes visible the exposed insoluble areas, while the hydrophilic phase dissolves away the unexposed soluble non-image or background areas. In this way, the visible image is made oleophilic or ink receptive and the background is made hydrophilic or ink repellent.
In our companion application, Ser. No. 064,322 filed Aug. 6, 1979, we describe the simultaneous chemical amplification of diazo sensitized plates using an anionic material in an ionizing reaction medium. This provides a water developable plate and eliminates the use of conventional solvent/resin containing lacquers. In the present invention we provide a lithographic substrate and plate that produces a visible image upon development without relying on a component of the developer to become adhered to the image area.